How to Solder PCB Boards: A Step-by-Step Guide

Knowing how to solder PCB boards is a crucial skill in electronics. You need it to mount components or save damaged circuit boards. You can also use it to restore legacy PCBs. This guide takes you through the soldering process. We also share helpful soldering tips.

What is PCB Soldering?

PCB soldering means using a low-melting metal alloy to form electrical and mechanical connections. We solder PCBs to attach components, repair broken joints and traces, or join parts.

The solder is usually either leaded or lead-free. The leaded type is banned in most countries, and its use is limited to high-reliability circuit boards, such as medical, automotive, military, and aerospace types.

Types of PCB Soldering

PCB soldering is either manual or automated. Each method has its place, depending on factors like the number of circuit boards. Let’s see when to use each and why.

Manual Soldering

The manual method involves using a heated rod and solder wire to attach components or create connections. Although slow and laborious, it reduces costs.

The manual method best suits repair needs or low-volume assembly. An example is when fixing a legacy circuit board or creating prototypes before full-fledged production.

Automated Soldering

Automated soldering uses complex equipment to reduce manual steps. It improves speed, making it the preferred method for high-volume projects.

The type of technology depends on the components being assembled. A reflow oven attaches surface mount (SMD) devices, while the wave method attaches through-hole parts.

How to Solder PCB Boards

Now that you understand what soldering means and its methods, it’s time to know how to do it step-by-step. We will also concern ourselves with manual method. Let’s begin with what you’ll need for the job.

Materials and Tools

Soldering is effective if you use the right materials and tools, in addition to following the correct steps. Ensure you have the following before beginning the process.

• Soldering Iron or Station: ensure it has an ergonomic handle and an adequate power rating of about 20-60 watts. It’s also helpful if it allows you to change tips.
  
• Solder Wire: choose between lead and lead-free solder based on your PCB type and the required solder joint quality.
  
• Solder Flux: it cleans the PCB surface and improves flow. The flux is available in various types, such as water-soluble and no-clean.
  
• Solder Wick or Braid: a strand of copper wires for absorbing excess solder.
  
• Desoldering Tools: they remove excessive or residual solder. They include a soldering wick/braid and an extraction pump.
  

You also need the following: a soldering iron stand, a tip cleaner (sponge or brass wool), a pair of tweezers, and a magnifying glass.

It’s also essential that you have a fume extractor to remove harmful gases. Other materials include a soldering iron holder and cardboard to catch dripping solder.

Board Preparation

• Clean the PCB using appropriate materials.
  
• You may use isopropyl alcohol and mild abrasion to remove stubborn contaminants.
  
• Take time to understand its schematics, too.
  

Soldering Steps

• Set up the workspace, ensuring you have everything within reach.
  
• Secure the PCB or place it on a flat surface where it will not move.
  
• Apply a small amount of solder to the tip, a process called tinning.
  
• Clean the tip with a wet sponge to remove excess flux.
  
• Insert the component leads in their holes on the PCB.
  
• You may need to bend the leads to secure the part in place.
  
• Hold SMT parts in place with soldering paste or another suitable method.
  
• Heat the iron to the appropriate temperature.
  
• Place the iron so it touches both the component lead and the PCB pad.
  
• Hold the iron in place for 2-3 seconds to heat the joint. 
  
• Touch the solder wire to the joint (not to the iron).
  
• The heat from the joint should instantly melt the solder, allowing it to flow and form a connection. 
  
• Allow the solder to flow into a concave shape.
  
• Remove the wire and the iron in that sequence.
  
• Let the joint cool and harden without disturbing the component.
  
• Inspect the joint for any signs of defects.
  
• Trim the lead or leads if necessary.
  
• Finally, clean the PCB to remove residues, including flux.
  

PCB Inspection and Testing After Soldering

Freshly soldered joints require proper inspection and testing. It ensures their electrical and mechanical integrity. The following are inspection and testing steps when soldering your circuit board.

Visual Inspection

Examine the formed joints for defects. Things to check out for include cold joints, improper flow, and short circuits, such as bridges. A quality leaded-solder joint is smooth and shiny. Note that lead-free solder joints will naturally appear duller or more matte. This is normal and does not indicate a bad joint.

You may use a magnifier if the joints are too small or if the board is a high-density type. Desolder defective joints and re-solder them using appropriate steps, taking care not to repeat earlier mistakes.

Continuity Tests

The formed connections must allow the flow of electricity as required. That includes not creating unwanted connections, such as short-circuits. Use a digital multimeter in continuity mode (it will beep if a connection exists) to test the joint. You should also check for shorts by testing continuity between adjacent, un-connected joints.

Common Problems When Soldering PCB Boards

You’re likely to encounter a few challenges when attaching or joining parts to a circuit board, especially if you’re a beginner. We compiled a list of the most common problems and their causes.

Excessive Heat

The soldering iron transfers too much heat to the board, resulting in component or pad damage. Ensure you understand the temperature limit of different electronic components and solder types.

Cold Joints

Cold joints form when the heat is too low and the alloy doesn’t fully melt. They also form if you remove the iron too early. A cold joint has a dull appearance. It’s also weak and may not last long.

Short Circuits

Short circuits mainly result from using too much solder. The excess solder forms unwanted connections, shorting circuits. Ensuring the right amount helps prevent the problem.

Component Shift

A component shifts if it lacks firm support. To prevent it, hold the part in place until the solder has hardened. You may bend the leads or use soldering paste to achieve that.

Mistakes to Avoid When Soldering PCB Boards

The quality of connections formed by the joints depends on how well you complete the process. Robust joints also last longer. Avoid these mistakes to ensure quality and long-lasting joints.

• Not using an appropriate tip: size the tip based on the available space and component type. Use a chisel tip for larger pads or a fine-point conical tip for small components.
  
• Applying too much solder or flux: only use the right amounts to avoid bridges when joining electrical components.
  
• Using too much or too little heat: Ensure you’re using the correct temperature level or setting for the alloy and component type. Some require more heat than others.
  
• Not following proper component placement sequence: always start with smaller, flat components before placing taller parts.
  
• Breathing toxic fumes: use a fume extractor throughout the process. Ensure the working area is adequately ventilated.
  

Conclusion

PCB soldering requires an understanding of its steps. You must be familiar with the necessary tools and know how to use them effectively. Regardless of whether you’re a DIY enthusiast or professional, the steps and tips in this guide should help you perfect the process. That, in turn, will improve the quality of formed joints, making them durable.